This invention relates generally to flat faceplate cathode ray tubes, and more particularly to tubes of this type which have a tensioned foil shadow mask. The invention also relates to a process for the manufacture of such tubes, including the heat treating of nickel-iron alloys to provide a desired combination of mechanical and magnetic properties necessary for effective operation of tensioned foil shadow masks. Also disclosed is a shadow mask formed from an improved alloy, and a front assembly containing such a mask.
Cathode ray tubes having flat faceplates and correspondingly flat tensioned foil shadow masks are known to provide many advantages over conventional cathode ray tubes having a curved faceplate and a curved shadow mask. A chief advantage of a flat faceplate cathode ray tube with tensioned mask is a greater electron beam power-handling capability, a capability which can provide greater picture brightness. The power-handling capabilitY of tubes having the conventional curved mask is limited due to the thickness of the mask (5 to 7 mils), and the fact that it is not mounted under tension. As a result, the mask tends to expand or "dome" in picture areas of high brightness where the intensity of electron beam bombardment, and consequently the heat, is greatest. Color impurities result when the mask expands toward the faceplate and the beam-passing apertures in the mask move out of registration with their associated phosphor dots or lines on the faceplate.
A tensioned foil mask when heated acts in a manner quite different from a curved, untensioned mask. For example, if the entire mask is heated uniformly, the mask expands and relaxes the tension. The mask remains planar and there is no doming and no distortion until the mask has expanded to the point that tension is completely lost. Just before all tension is lost, wrinkling may occur in the corners. When small areas of a tensioned foil mask are differentially heated, the heated areas expand and the unheated areas correspondingly contract, resulting in only small displacements within the plane of the mask. However, the mask remains planar and properly spaced from the faceplate and, consequently, and any color impurities are unnoticeable.
The mask must be supported in tension in order to maintain the mask in a planar state during operation of the cathode ray tube. The amount of tension required will depend upon how much the mask material expands upon heating during operation of the cathode ray tube. Materials with very low thermal coefficients of expansion need only a low tension. Generally, however, the tension should be as high as possible because the higher the tension, the greater the heat incurred, and the great the electron beam current that can be handled. There is a limit to mask tension, however, as too great a tension can cause the mask to tear.
The foil mask may be tensioned in accordance with known practice. A convenient method is to thermally expand the mask by means of heated platens applied to both sides of the foil mask. The expanded mask is then clamped in a fixture and, upon cooling, remains under tension. The mask may also be expanded by exposure to infrared radiation, by electrical resistance heating, or by stretching through the application of mechanical forces to its edges.
In addition to having the composition as described herein, after heat treatment and slow cooling according to the invention, a foil formed from the alloys will have a unique combination of mechanical, thermal and magnetic properties that makes it uniquely suited for use as a tensioned foil shadow mask. The alloy, in as-cast or in treated form, must have adequate ductility to permit it to be hot or cold rolled to a foil having a thickness of less than 2 mils, preferably to a thickness of 1 mil, or even as thin as 0.5 mil. A 1 mil thick foil when rolled will typically have a reduction in area of at least 0.8 percent and preferably at least 1.0 percent elongation. To withstand the forces incident to the tensing operation, the mask material should have a yield strength above about 80 ksi and preferably above about 100 ksi (0.2 percent offset). The mask material should also be able to withstand a tension load of at least about 25 Newton/centimeter, preferably at least about 65 Newton/centimeter. The mask material should also have a thermal coefficient of expansion that is not substantially less than that of the glass of the faceplate.
In addition to the mechanical properties described, the mask material must have a particular combination of magnetic properties. In this connection, it is important that the mask material have as high a permeability as is possible while maintaining the necessary mechanical properties. The permeability should be at least about 6,000, preferably at least about 10,000, and most desirably in excess of 60,000. A maximum coercivity is desirably below about 1.0 oersted and preferably is below about 0.5 oersted.